The secondary display content worked fine. Screenshots of the window produced no artifacts, but the rendered display looked terrible. The crap render persisted when I dragged the main Aperture window from the primary display to the secondary display, so it isn't a monitor or video card issue.

In frustration, I disconnected the second monitor from the NVIDIA card and re-connected it to the second DVI-D port on the ATI Radeon X1900 XT. Everyone seems to work now.

I kept the stupid NVIDIA card just so I could use it with a second monitor. Now, it appears better to have a single card than to attempt to use two.

Come on, Apple! Get your shit together. You only have to deal with, what, 4 video cards drivers on the Mac Pro? You'd think they would work flawlessly.